The genetics of vacuoles Biogenesis and function in plant cells

Vacuoles are endomembrane compartments that in plants play pivotal roles in a plethora of cellular processes. The acidification of the vacuolar lumen drives many of this functions by, for instance, energizing transporters on the vacuolar membrane. In flowers, vacuolar hyper-acidification also determines petal color, while the very same mechanism is responsible for the acidification of juice cells in the flesh of Citrus fruits. Vacuoles of different dimensions and functions coexist in a specific cell-types. In epidermal cells of petunia petals, small vacuoles, called vacuolinos, are described. This opens many questions on the mechanisms of evolution, biogenesis, and function of these novel compartments. Vacuolinos appear to act as “gatekeepers” or sorting stations, which prevent specific proteins from reaching the central vacuole. Among the proteins getting halted in the vacuolinos, and therefore not reaching the central vacuole where pigments are stored, is FADING, an enzyme responsible for anthocyanin destabilization and color loss. Vacuolinos have therefore a role in petal color stability. The small GTPase RAB5a, that is very conserved throughout plant and animal species, but was lost in Arabidopsis and related species, mediates the formation of vacuolinos by promoting the fusion of pre-vacuolar compartments. Also PH7, a monosaccharide-sugar transporter, is required for the formation of vacuolinos and its traffic to the central vacuole, as shown by the loss of these compartments and defects in protein sorting in flowers of ph7 mutants. The findings reported in this thesis extend our knowledge on vacuoles biology and pave the way for further research.